The recent trend for portable devices has increased the needs and requirements for high energy density and high power density rechargeable batteries. High energy density and high power density are also important criteria for batteries used for electric or hybrid vehicles.
Nickel hydroxide has been used for years as an active material for the positive electrode of alkaline electrochemical cells. Examples of such nickel-based alkaline cells include nickel cadmium (Ni—Cd) cells, nickel-iron (Ni—Fe) cells, nickel-zinc (Ni—Zn) cells, and nickel-metal hydride (Ni—MH) cells. The energy density of nickel-based electrochemical cells may be increased by closely packing the nickel hydroxide active material into an electrically conductive substrate such as a porous foam. However, there are limitations on the amount of pressure used to increase packing density. Application of too much pressure causes expansion of electrode plates and compresses the separators placed between the positive and negative electrodes. The overcompression of the cells limit the wettability as well as the amount of electrolyte in separators by squeezing out the absorbed electrolyte, which in turn deteriorates the performance of these cells.
In general, nickel-metal hydride (Ni—MH) cells utilize a negative electrode comprising a metal hydride active material that is capable of the reversible electrochemical storage of hydrogen. Examples of metal hydride materials are provided in U.S. Pat. Nos. 4,551,400, 4,728,586, and 5,536,591 the disclosures of which are incorporated by reference herein. The positive electrode of the nickel-metal hydride cell comprises a nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical current across a Ni—MH cell, the Ni—MH material of the negative electrode is charged by the absorption of hydrogen formed by electrochemical water discharge reaction and the electrochemical generation of hydroxyl ions:
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The charging process for a nickel hydroxide positive electrode in an alkaline electrochemical cell is governed by the following reaction:

After the first charge of the electrochemical cell, the nickel hydroxide is oxidized to form nickel oxyhydroxide. During discharge of the electrochemical cell, the nickel oxyhydroxide is reduced to form beta nickel hydroxide as shown by the following reaction:

The charging efficiency of the positive electrode and the utilization of the positive electrode material is affected by the oxygen evolution process which is controlled by the reaction:2OH—→H2O+½ O2+2e−  (4)During the charging process, a portion of the current applied to the electrochemical cell for the purpose of charging, is instead consumed by a parallel oxygen evolution reaction (4). The oxygen evolution reaction generally begins when the electrochemical cell is approximately 20–30% charged and increases with the increased charge. The oxygen evolution reaction is also more prevalent with increased temperatures. The oxygen evolution reaction (4) is not desirable and contributes to lower utilization rates upon charging, can cause a pressure build-up within the electrochemical cell, and can upon further oxidation change the nickel oxyhydroxide into its less conductive forms. One reason both reactions occur is that their electrochemical potential values are very close. Anything that can be done to widen the gap between them (i.e., lowering the nickel reaction potential in reaction (2) or raising the reaction potential of the oxygen evolution reaction (4)) will contribute to higher utilization rates. It is noted that the reaction potential of the oxygen evolution reaction (4) is also referred to as the oxygen evolution potential.
Furthermore, the electrochemical reaction potential of reaction (4) is highly temperature dependent. At lower temperatures, oxygen evolution is low and the charging efficiency of the nickel positive electrode is high. However, at higher temperatures, the electrochemical reaction potential of reaction (4) decreases and the rate of the oxygen evolution reaction (4) increases so that the charging efficiency of the nickel hydroxide positive electrode drops.
One way to increase the electrochemical potential of equation (4) is by adding certain additives with the nickel hydroxide active material when forming the positive electrode material. U.S. Pat. Nos. 5,466,543, 5,451,475, 5,571,636, 6,017,655, 6,150,054, and 6,287,726 disclose certain additives which improve the rate of utilization of the nickel hydroxide in a wide temperature range. The present invention discloses an improved additive which enhances performance of the positive electrode by reducing the resistance within the nickel electrode and simultaneously increasing the oxygen evolution potential.